The goal of this research project is to define the characteristics of single nephron function during post-natal maturation by using the technique of micropuncture. The animal used for these experiments is the guinea pig which, like the human but unlike most of the other mammals, has a fully developed cortex at birth. The initial objective was to determine the mechanism accounting for the increase in GFR following birth and to describe the pattern of this increase. We were able to demonstrate that the increase in effective filtration pressure accounts only in part for the increase in GFR and that the centrifugal pattern which characterizes the development of the kidney during intrauterine life continues after birth. Presently we are involved in experiments designed to measure the permeability characteristics of the renal tubule during maturation. Using as a marker radioactive inulin, known amounts of various substances with different molecular weights are injected into the early proximal tubules and collected in the final urine. The ratio between these substances and inulin provides an estimate of the permeability of the tubular wall. We are also testing the intrinsic capacity of the proximal tubule of the growing animal to reabsorb sodium. A stopped- flow microinjection-recollection technique employing rafinose is used for this purpose.